Technical stuff on electric motor (worth the reading)

[Mister-T]

A friend of mine posted on his blog a good article about electric motor, it's not too much technical. Original blog is there http://blogs.nofrag.com/DarkRedemptor/

My knowledge about electric motor brushed and brushless

I) The solenoid and the electromagnet
The very first electromagnet is the solenoid.
This electro magnet is fair easy to understand for one solenoid, the magnetic field is proportional to the current applied.
B=(N*I/h)* µ0
where µ0 = 4*PI * 10 exp(-7)
h the length of solenoid
N the number of turns

Some people have found that using a Ferric core give more magnetic field, the formula don't give a linear correlation between magnetic and current :
The magnetic field B rise quickly with current compare as a solenoid.
You can find more explain here
The formula is B=(N*I)/(Lcore/µ +Lgap/µ0)

where µ0 = 4*PI * 10 exp(-7)
µ depend of B that why the system is non linear
Lcore the length of the bobinage around the core
Lgap the length of the bobinage outside the core
N is the number of turns

In our electrical motor, all the wound are around a ferric core (it's not really true because core are laminated) so you can simplfy the equation by :
B=(N*I)/(Lcore)*µ

Ok so things that made our electrical motor have torque is turn (the more you have turn the more you have torque) and current (the more you have current the more you have torque)

Easy no ?
But wait, we don't use current to drive our motor but voltage, no ? As you know U=R*I so current and voltage arelinear depending to resistance, here we speak about impedance because we can see the solenoid as a coil.
Note, with ferric core the impedance of the solenoid go down : the more the ferric core you have the lower impedance you have.

Let continuous...



II ) Brushed motor
First : Every people that find stock motor have less torque than a 10 turn motor are wrong.
Remember the torque depend on the motor turns and the current. The voltage is fixed at 7V due to battery internal resistance (in true the voltage can vary between 5V to 7V even with super matched pack). Current maybe Higher in the 10 turn but with ferric core, the current in the stock motor motor is also quite high (around 100A for each !! but little more for the 10turns)
Ok but the stock as more turn than the 10 turn modified motor so it have more torque.

Don't trust me ?
Take a car and with the same ratio try a stock motor agaisnt modified motor. Thath why modified class use shorter ratio (little pinion big spur gear) than stock (big pinion little spur gear)

so why 10T modified is more powerfull than a stock ? Because of power of couse ;-)
What is Power in motor (or gas engine) ?
Power = torque * angular_speed
where angular_speed is in rad/s
power is in Watt
Torque in in N.m

Translate to our better know unit
power = torque * 2*PI * RPM * 60

I hope you understand my think, the 10T modified is more powerfull than the stock because it had more RPM.

Remember your solenoid is like a coil, the coil became a thing know as schok coil because of is ferric core. It's a filter that simply cut off high frequency.
Just take a moment to calculate something :
Our motor work at 30'000 RPM (a good modified can go faster) we have 3 coil in our motor (three pole) but one coil must make a complete revolution for one Perdiod
So the current frequency in one coil is :
f = RPM*60
Because we have 60 second in one minute, so in our example f = 30'000*60 = 1'800'000 Hz, you read right, you have 1.8 MHz this frequency is simply cut off by the schok coil.

So 10T modified motor run faster than stock motor but have less torque. The 10T motor have more power simply because it can run faster.

In truth there is not a beautifull sinusoid that pass in the coil, but it's more like a square signal (at 1.8MHz in our exemple) with all high frequency harmonics behinds this kind of signal (It's exclusively for the brushed motor)


There is another limitation, remember, to have a lot of torque you need a big ferric core. I sayed the core is laminated, it's not the best thing to do for the strongest magnet field.
In fact at high frequency the magnetic field generate Foucault current (those currents are used in induction coocker) there are located in the core (ie not in the coil), laminated core limit those current (should be seen as many littles cores working in parallel).
For explain, each plate that form the core of our motors are isolated each other with some kind of varnishes, that is one reason that balancing rotor with epoxy is better than drilling it.
The Foucault current rises with RPM and generates heat, they are also create magnetic field opposed to the change of the magnetic fields (Imagine when the main current say to one coil "switch from north to south", the Foucault current say "stay in north condition").
In other word, the rise of the current, ie if we use less turn on bobinage or if we use higher voltage( 8.4V pack for example) will not give us more RPM, but certainly a little more RPM and more heat.

So in brushed it seems that 10T is the best we can do (following the rules of course). The 8T motor still exist but are only use with 5 cell pack and equivalent to 10T with 6 cell pack but with less torque (a stupidity dicted by commercial interest of certain matcher I think, but nevermind)

Brushed motors are particulary sensitive to Foucault current because of comutator give a voltage that look like square (with very high frequency harmonic and high frequency mean high Foucault current), it's one of the factor that made the brushless so interrested.


Single, Double, Triple, Quadruple, Quintuple, this term mean we will use 1 cable to made wounding or 2 cables in parallel to made wouding or 3 cables in parallel to made wouding etc...
The way is the following for this example we'll take a 12T mod motor, physicaly your rotor have a certain limit (diameter and longer).
So a 12T single wire had a higher section than the sum of the 2 wire section of a 12 double etc...
On the contrary a 12T quintuple are more wire surface than a 12T quadruple etc...

How work a cable ? With low frequency voltage (ie at low RPM), the more wire section you have the higher current you can pass. Remember current = torque. So the 12T Single are more startup torque than 12T Quintuple.
With an high frequency voltage (ie at high RPM), the more wire surface you have the higher current you can pass (it's called skin effect). So the 12T Quintuple are more high end torque than 12T Single (that also mean more high end power).

The skin effect is not predominant for the use of cylindrical brush, again it seems more commercial. The current is continuous on the brush. Even though like the cable the more section you have, the more current you pass.


For concluding Brushed motor Let summarize :
- High turns give more torque than Low turns
- Low turns give more speed than High turns
- There are a speed limit of any brushed due to Foucault currents that oppose the changing of magnetic fields.
- less torque < Quintuple < Quadruple < Triple < Double < Single < more torque
- less high end RPM < Single < Double < Triple < Quadruple < Quintuple < high end RPM

To finish brushed part : Single are used in DTM (8T for 5 cell, 10T for 6 cell), Double are used in 4WD offroad (Single may also be used depend on the track) Triple are used in 2WD offroad (Double may also be used depend on the track)

Some informations



III) Brushless motor
The Holy Grall electrical motor ? (actually not but in RC car we have 10 years behind the flying-model, we will see later)
A brushless work with the same law than brushed but with two bonus. Remember, on brushed motor Foucault currents limits the RPM.
On a Brushless, the controller generate the sinusoid (In true 3 sinusoid are generated, 1 for each coil) , there are less hamonic and the motor can go faster.
This also permit to rate the motor in kV because they have a good linearity so 5000kV will signifie 5'000 RPM at 1V, 10'000 RPM at 2V, 15'000 RPM at 3V, etc...
The Foucault current are not inexistant, but at high frequency there only give heat, in fact the brushless controller drive the coil to anhilate opposition of changing the magnetic fields. It requiers more current, heat up more both controller and the motor. (In modern Brushed /Speedo combo only the motor heat up)
In other word the most current drain your voltage source and controller can take the faster you can go. If your pack is in bad shape you loose high end RPM, if you controller can not manage a lot of current you also loose RPM.
Typicaly the brushless are faster than the brushed by construction, that mean they need both good battery pack and good controller. (a matched pack and a controller that manage at least 100A/phase is the neccesary, more is better)

In RC car application the brushless are usualy wounded in double or more, with two kind of schem : star or delta. The delta gives low torque at low rpm, but can give higher top rpm (higher current drain) when the star gives high torque at low rpm, but not as high top rpm.
The Delta wounded is not autorized in ROAR but legal in EFRA, Delta wound offer is best perspective for offroad where the Y-wounded brushless have a little too much torque, and off road track does not permit easily to accelerate (low grip).
Note the delta is use much more current. Actually there are 2 delta wounded motor Tekin Redline and GM Racing. More information

NEXT PART 2

[Mister-T]

The brushless have more torque than brushed for 2 reasons :
- Even you have smaller permanent magnet (with theoricaly less magnetic field), the rotor is mush more lighter than a brushed rotor (iron and copper are heavy)
- In RC car the brushless has the autorization to use Neodymium Magnet when brushed simply use Ferrite magnet (the rules are dicted by two manufacturer for ROAR and EFRA... There was a time where dragsters used brushed motor with Neodymium magnet)

The other interest about brushless is the reliability, no comutator, no brush. Less maintenance time = more time to fun.

Sensor or not Sensor ?
The Sensored have better startup (I don't speak about torque) but in high end RPM the Sensorless is faster (because of control loop feedback that take time in sensored case)


For concluding Brushless motor Let summarize :
- Theorical like the brushed
- Linear relation between RPM and voltage
- There are no speed limit due to Foucault currents but speed limit due to the controller and battery ability to drive/generate high current drain.
- They have more torque due to their Neodymium Magnet
- The torque is given at low RPM (Y-wound) or high RPM (Delta wound)
- The sensor will give a bonus when you start a round (the best way I think is to not use sensor when the motor reach 2000 RPM)

IV)timming
I put this in other chapter because it's aviable on every motor (even sensorless).
In the motor (brushed or brushless) the magnetic field of one coil is changed (its current goes through 0) when the coil is perpendicular to North-South line of the permanent magnet (either stator or rotor)
The main Idea is to apply the change of the magnetic field of coil before this event, by this technique you delay the Foucault current in a little high RPM.
The Timing angle can vary from -45° (useless) to 45° but it's pratical impossible to put so much timing angle. When you put timing angle keep in mind that your coil will switch when it's in a field of permanent magnet. A coil became a generator a send current in normaly input that will heat up either controller (brushed or brushless) and the motor.
To my mind there are a stock class in US (around mid 90's) that work with 36° timing stock motor.
With current stock motor it's not a really good idea to do that (17T in France).


V)Future
Just one word about a kind of brushless called outrunner, the rotor (with permanent magnet) is outside and rotate aroud the stator.
The idea behind is to give more torque due to the bigger moment the permanent magnet give (permanent magnet is one key of torque)
Already used in Air model, not in car model (you can even find some for your car but not legal ROAR or EFRA) They are probably also too dangerous for marshal turn (they don't have "encapsuled" rotor).

To our future engineer, the R/D on electrical motor are not finish there are a lot of dark part that no one can explain.

I hope you enjoy this explain, I try to not put equation, because I'm not good to resolve mathematics, but I bet the main electrical motor concept are here.[/QUOTE]

[crazyjr]

I'd really like to see a Neo brushed motor against a brushless, to see if the efficicncies are really that far off what is the current norm.

I really like your signature, Steve Neu has also said the same thing when asked about the Novak and other sensored designs